The present invention relates to plastic films and, in particular, to films having improved anti-fog properties.
In the development of films for the packaging of various articles and materials especially foodstuffs, major efforts have been directed to providing films with low permeability to avoid moisture loss from the article or contamination of the article by undesired ambient vapors. Another problem in the use of packaging films, especially for the wrapping of products of high moisture content, is the tendency of moisture within the package to condense as droplets or as a fog on the inside surface of the package especially when the package is refrigerated. The resulting packaged product is unattractive and, the product being displayed is obscured from view by droplets. This characteristic defeats in part the purpose for using a transparent wrapping material.
Attempts have been made in the past to reduce the fogging tendency by applying hydrophilic coatings to the surface of the film in order to overcome the natural hydrophobic characteristics of the film surface and to cause the condensed moisture to wet-out on the film surface, thereby eliminating individual droplet formations and resulting fogging. However, such coatings usually detract from other desirable properties of the film such as gloss and heat sealability. Moreover, such coatings are water soluble materials which wash away when moisture condenses on the coated films.
For example, U.S. Pat. No. 5,030,280 discloses an anti-fog coating of polymethyl methacrylate or polycarbonate which may be applied to packaging films by means of a cloth, spraying or the like. Anionic surfactants described therein have a hydrophilic-lipophilic balance or HLB value of from 2 to less than 13. The coating described therein, like others in prior art, readily washes away.
An anti-fog coating composition with good heat sealability properties which does not wash away easily is highly desirable.
Accordingly, it is an object of the present invention to provide an anti-fog coating which is long lasting and does not wash away easily. It is a further object of the present invention to provide an anti-fog composition which can be readily and effectively applied to oriented substrates at selected times during processing.
It is a further object of the invention to provide an anti-fog composition which when applied to plastic films enhances the heat seal strength of the coated films.